Sudden infant death syndrome (SIDS), also known as cot death or crib death, is the sudden unexplained death of a child less than one year of age.[1] Diagnosis requires that the death remains unexplained even after a thorough autopsy and detailed death scene investigation.[2] SIDS usually occurs during sleep.[3] Typically death occurs between the hours of 00:00 and 09:00.[4] There is usually no evidence of struggle and no noise produced.[5]

The exact cause of SIDS is unknown.[6] The requirement of a combination of factors including a specific underlying susceptibility, a specific time in development, and an environmental stressor has been proposed.[3][6] These environmental stressors may include sleeping on the stomach or side, overheating, and exposure to tobacco smoke.[6] Accidental suffocation such as during bed sharing or from soft objects may also play a role.[3][7] Another risk factor is being born before 39 weeks of gestation.[8] SIDS makes up about 80% of sudden and unexpected infant deaths (SUIDs), with other causes including infections, genetic disorders, and heart problems. While child abuse in the form of intentional suffocation may be misdiagnosed as SIDS, this is believed to make up less than 5% of cases.[3]

The most effective method of reducing the risk of SIDS is putting a child less than one year old on their back to sleep.[8] Other measures include a firm mattress separate from but close to caregivers, no loose bedding, a relatively cool sleeping environment, using a pacifier, and avoiding exposure to tobacco smoke.[9]Breastfeeding and immunization may also be preventive.[9][10] Measures not shown to be useful include positioning devices and baby monitors.[9][10] Evidence is not sufficient for the use of fans.[9] Grief support for families impacted by SIDS is important, as the death of the infant is sudden, without witnesses, and often associated with an investigation.[3]

Rates of SIDS vary nearly tenfold in developed countries from one in a thousand to one in ten thousand.[3] Globally it resulted in about 15,000 deaths in 2013 down from 22,000 deaths in 1990.[11] SIDS was the third leading cause of death in children less than one year old in the United States in 2011.[12] It is the most common cause of death between one month and one year of age.[8] About 90% of cases happen before six months of age, with it being most frequent between two months and four months of age.[3][8] It is more common in boys than girls.[8]

SIDS is a diagnosis of exclusion and should be applied to only those cases in which an infant's death is sudden and unexpected, and remains unexplained after the performance of an adequate postmortem investigation, including:

After investigation, some of these infant deaths are found to be caused by accidental suffocation, hyperthermia or hypothermia, neglect or some other defined cause.[13]

Australia and New Zealand are shifting to the term "sudden unexpected death in infancy" (SUDI) for professional, scientific, and coronial clarity.

The term SUDI is now often used instead of sudden infant death syndrome (SIDS) because some coroners prefer to use the term 'undetermined' for a death previously considered to be SIDS. This change is causing diagnostic shift in the mortality data.[14]

SIDS has a 4-parameter lognormalage distribution that spares infants shortly after birth — the time of maximal risk for almost all other causes of non-trauma infant death.

By definition, SIDS deaths occur under the age of one year, with the peak incidence occurring when the infant is at 2 to 4 months of age. This is considered a critical period because the infant's ability to rouse from sleep is not yet mature.[3]

The cause of SIDS is unknown. Although studies have identified risk factors for SIDS, such as putting infants to bed on their stomachs, there has been little understanding of the syndrome's biological process or its potential causes. The frequency of SIDS does appear to be influenced by social, economic, and cultural factors, such as maternal education, race or ethnicity, and poverty.[16] SIDS is believed to occur when an infant with an underlying biological vulnerability, who is at a critical development age, is exposed to an external trigger.[3] The following risk factors generally contribute either to the underlying biological vulnerability or represent an external trigger:

SIDS rates are higher for infants of mothers who smoke during pregnancy.[17][18] SID correlates with levels of nicotine and derivatives in the infant.[19] Nicotine and derivatives cause significant alterations in fetal neurodevelopment.[20]

Placing an infant to sleep while lying on the stomach or the side increases the risk.[9] This increased risk is greatest at two to three months of age.[9] Elevated or reduced room temperature also increases the risk,[21] as does excessive bedding, clothing, soft sleep surfaces, and stuffed animals.[22]Bumper pads may increase the risk and, as there is little evidence of benefit from their use, they are not recommended.[9]

Sharing a bed with parents or siblings increases the risk for SIDS.[23] This risk is greatest in the first three months of life, when the mattress is soft, when one or more persons share the infant's bed, especially when the bed partners are using drugs or alcohol or are smoking.[9] The risk remains, however, even in parents who do not smoke or use drugs.[24] The American Academy of Pediatrics thus recommends "room-sharing without bed-sharing", stating that such an arrangement can decrease the risk of SIDS by up to 50%. Furthermore, the Academy recommended against devices marketed to make bed-sharing "safe", such as in-bed co-sleepers.[25]

SIDS rates decrease with increasing maternal age, with teenage mothers at greatest risk.[17] Delayed or inadequate prenatal care also increases risk.[17] Low birth weight is a significant risk factor. In the United States from 1995 to 1998, the SIDS death rate for infants weighing 1000–1499 g was 2.89/1000, while for a birth weight of 3500–3999 g, it was only 0.51/1000.[28][29]Premature birth increases the risk of SIDS death roughly fourfold.[17][28] From 1995 to 1998, the U.S. SIDS rate for births at 37–39 weeks of gestation was 0.73/1000, while the SIDS rate for births at 28–31 weeks of gestation was 2.39/1000.[28]

Anemia has also been linked to SIDS[30] (note, however, that per item 6 in the list of epidemiologic characteristics below, extent of anemia cannot be evaluated at autopsy because an infant's total hemoglobin can only be measured during life.[31]). SIDS incidence rises from zero at birth, is highest from two to four months of age, and declines toward zero after the infant's first year.[32] Baby boys have a ~50% higher risk of SIDS than girls.[33]

Genetics plays a role, as SIDS is more prevalent in males.[34][35] There is a consistent 50% male excess in SIDS per 1000 live births of each sex. Given a 5% male excess birth rate, there appears to be 3.15 male SIDS cases per 2 female, for a male fraction of 0.61.[34][35] This value of 61% in the US is an average of 57% black male SIDS, 62.2% white male SIDS and 59.4% for all other races combined. Note that when multiracial parentage is involved, infant race is arbitrarily assigned to one category or the other; most often it is chosen by the mother. The X-linkage hypothesis for SIDS and the male excess in infant mortality have shown that the 50% male excess could be related to a dominant X-linked allele, occurring with a frequency of 1⁄3 that is protective of transient cerebral anoxia. An unprotected male would occur with a frequency of 2⁄3 and an unprotected female would occur with a frequency of 4⁄9.

About 10 to 20% of SIDS cases are believed to be due to channelopathies, which are inherited defects in the ion channels which play an important role in the contraction of the heart.[36]

Vaccinations do not increase the risk of SIDS; contrarily, they are linked to a 50% lower risk of SIDS.[40][41]

A 1998 report found that antimony- and phosphorus-containing compounds used as fire retardants in PVC and other cot mattress materials are not a cause of SIDS.[42] The report also states that toxic gas can not be generated from antimony in mattresses and that babies suffered SIDS on mattresses that did not contain the compound.

A set of risk factors SIDS has been identified with: seasonality: winter maximum, summer minimum; increasing SIDS rate with live birth order; low increased risk of SIDS in subsequent siblings of SIDS; apparent life-threatening events (ALTE) are not a risk factor for subsequent SIDS; SIDS risk is greatest during sleep.[43]

For example, an infant with MCAD deficiency could have died by "classical SIDS" if found swaddled and prone with head covered in an overheated room where parents were smoking. Genes indicating susceptibility to MCAD and Long QT syndrome do not protect an infant from dying of classical SIDS. Therefore, presence of a susceptibility gene, such as for MCAD, means the infant may have died either from SIDS or from MCAD deficiency. It is currently impossible for the pathologist to distinguish between them.

A 2010 study looked at 554 autopsies of infants in North Carolina that listed SIDS as the cause of death, and suggested that many of these deaths may have been due to accidental suffocation. The study found that 69% of autopsies listed other possible risk factors that could have led to death, such as unsafe bedding or sleeping with adults.[51]

Several instances of infanticide have been uncovered where the diagnosis was originally SIDS.[52][53] Estimate of the percentage of SIDS deaths that are actually infanticide vary from less than 1% to up to 5% of cases.[54]

Some have underestimated the risk of two SIDS deaths occurring in the same family and the Royal Statistical Society issued a media release refuting this expert testimony in one UK case in which the conviction was subsequently overturned.[55]

A number of measures have been found to be effective in preventing SIDS including changing the sleeping position, breastfeeding, limiting soft bedding, immunizing the infant and using pacifiers.[9] The use of electronic monitors has not been found to be useful as a preventative strategy.[9] The effect that fans might have on the risk of SIDS has not been studied well enough to make any recommendation about them.[9] Evidence regarding swaddling is unclear regarding SIDS.[9] A 2016 review found tentative evidence that swaddling increases risk of SIDS, especially among babies placed on their stomachs or side while sleeping.[56]

In colder environments where bedding is required to maintain a baby's body temperature, the use of a "baby sleep bag" or "sleep sack" is becoming more popular. This is a soft bag with holes for the baby's arms and head. A zipper allows the bag to be closed around the baby. A study published in the European Journal of Pediatrics in August 1998[62] has shown the protective effects of a sleep sack as reducing the incidence of turning from back to front during sleep, reinforcing putting a baby to sleep on its back for placement into the sleep sack and preventing bedding from coming up over the face which leads to increased temperature and carbon dioxide rebreathing. They conclude in their study, "The use of a sleeping-sack should be particularly promoted for infants with a low birth weight." The American Academy of Pediatrics also recommends them as a type of bedding that warms the baby without covering its head.[63]

A large investigation into diphtheria-tetanus-pertussis vaccination and potential SIDS association by Berlin School of Public Health, Charité – Universitätsmedizin Berlin concluded: "Increased DTP immunisation coverage is associated with decreased SIDS mortality. Current recommendations on timely DTP immunisation should be emphasised to prevent not only specific infectious diseases but also potentially SIDS."[64]

Many other studies have also reached conclusions that vaccinations reduce the risk of SIDS. Studies generally show that SIDS risk is approximately halved by vaccinations.[65][66][67][68][69]

Families who are impacted by SIDS should be offered emotional support and grief counseling.[70] The experience and manifestation of grief at the loss of an infant are impacted by cultural and individual differences.[71]

Globally SIDS resulted in about 22,000 deaths as of 2010, down from 30,000 deaths in 1990.[72] Rates vary significantly by population from 0.05 per 1000 in Hong Kong to 6.7 per 1000 in American Indians.[73]

SIDS was responsible for 0.54 deaths per 1,000 live births in the US in 2005.[28] It is responsible for far fewer deaths than congenital disorders and disorders related to short gestation, though it is the leading cause of death in healthy infants after one month of age.

SIDS deaths in the US decreased from 4,895 in 1992 to 2,247 in 2004.[74] But, during a similar time period, 1989 to 2004, SIDS being listed as the cause of death for sudden infant death (SID) decreased from 80% to 55%.[74] According to John Kattwinkel, chairman of the Centers for Disease Control and Prevention (CDC) Special Task Force on SIDS "A lot of us are concerned that the rate (of SIDS) isn't decreasing significantly, but that a lot of it is just code shifting".[74]

In 2013, there are persistent disparities in SIDS deaths among racial and ethnic groups in the U.S. In 2009, the rates of death range from 20.3 for Asian/Pacific Islander to 119.2 for American Indians/Alaska Native. African American infants have a 24% greater risk of having a SIDS related death [75] and experience a 2.5 greater incidence of SIDS than in Caucasian infants.[76] Rates are per 100,000 live births and enable more accurate comparison across groups of different total population size.

Research suggests that factors which contribute more directly to SIDS risk—maternal age, exposure to smoking, safe sleep practices, etc.—vary by racial and ethnic group and therefore risk exposure also varies by these groups.[3] Risk factors associated with prone sleeping patterns of African American families include mother’s age, household poverty index, rural/urban status of residence, and infant’s age. More than 50% of African American infants were placed in non-recommended sleeping positions according to a study completed in South Carolina.[77] Cultural factors can be protective as well as problematic.[78]

^Brathwaite-Fisher, T, Bronheim, S. Cultural Competence and Sudden Infant Death Syndrome and Other Infant Death: A Review of the Literature from 1990–2000. National Center for Cultural Competence, Georgetown University Center for Child and Human Development 2001. DOI: http://gucchd.georgetown.edu/72396.html